The present invention relates to airborne pulsed radars which detect target threats, in general, and more particularly, to a solid-state pulsed radar transmitter, especially for use therein, which includes pulse shaping control to reduce the spectral sidelobe energy being transmitted.
Presently, pulsed type radars are being included on military aircraft, like fighters and bombers, for example, to detect the presence of target threats. In general, military aircraft of this type fly in relatively close formation and as a result, their radars are operating in close proximity to one another. Recently, especially with the advent of solid-state transmitters, the shape of the transmitted radar pulse has been made rectangular to maximize the total effective radiated radar power. Moreover, the transmitter designs which include high-efficiency class C amplifiers have successfully achieved very sharp edged short rise and fall times for the rectangularly transmitted energy pulses. While these accomplishments have maximized output radar power, they have also increased the spectral bandwidth of the transmitted pulses providing for excessive energy in the spectral sidelobes about the transmitted frequency. In turn, this excessive spectral bandwidth energy being transmitted has at time degraded the performance of the other aircraft radars operating in close proximity by causing false alarms due to mutual interference between radar sets. That is, the radar receiver of an aircraft in formation may receive the sidelobe energy level of a signal transmitted by a neighbor aircraft in formation and falsely identify the received transmitted energy signal as a target, thereby causing a false alarm to occur.
Through analysis, it has been identified that the excessive bandwidth of energy in the transmission spectrum is primarily a result of the sharp inflection points of the rectangular shaped pulses being transmitted. In fact, if the pulse waveform could be generated with no finite discontinuities, as much as a 60 db/decade sidelobe rolloff in the spectral bandwidth associated therewith may be achieved. One example of a pulse waveform having a continuously rising edge is expressed by the following formula: EQU Pmax[1- cos (.pi.t/T.sub.rise)]
From the above, it appears that to improve the performance of radar sets operating in close proximity to one another, some sort of pulse shaping control is of paramount importance, especially a control which strives to maintain power efficiency. Such a control is described here below in a solid-state transmitter circuit embodiment which utilizes class C amplifiers in the pulse forming operation thereof.